A composition is the actual intellectual property of a song
A composition is the collection of notes, lyrics, harmonies, and melodies that make up a song or instrumental. The owner of the composition has publishing rights, while the owner of the actual recording of the song, often a record label, owns the master rights.
It is easiest to think of a composition as the sheet music and written lyrics, not the actual recording of the song.
An example of a composition is Mozart’s Requiem, which was written, quite fittingly, on his deathbed and later completed by Franz Xaver Süssmayr after he passed. Mozart’s Requiem is also an example of a song in the public domain.